I'm actually not sure if we really need two separate modes, 'Active' and 'Selected'. Maybe we should merge them into one ('Active/Selected')? @venomgfx, @Jonathan Williamson (carter2422), @Paweł Łyczkowski (plyczkowski), any opinion?

The reason for that change for me was the need to really only see selected objects. I use it currently on a big project at work. It is a representation whats going on inside blender and very useful for many operators that depend on selection state and also for python debugging purposes. An alternative solution maybe would be to make it more clear of whats selected in the outliner in general by changing how selection state is represented. The current method/theme does have some drawbacks im some cases - like the camera icon.

@Julian Eisel (Severin) I guess I can see a scenario where one would want a window that only displays the active object, along with Selectable/Renderable etc buttons, for quick access.

Active object was added there for a reason, anyone checked history what was the reasoning?

As showing all invisible objects -- i don't think it's a really good thing. In the viewport you usually use layers to limit amount of objects you're working on. Current logic helps keeping outliner in a usable state for complex scenes. If you change this, then all of a sudden you'll have objects appearing in the outliner which you never want to do anything with them.

Agree having active-unselected included with selected is strange. (Think this change can go directly into master).

But would keep selected behavior as-is (meaning "Visible Selected")
Tooltip can note that "Selected means visible-selected"

@Julian Eisel (Severin), Agree having 'Active' is a bit odd, The only reason I can think of it being useful is if you're dealing with complex hierarchies and you want to isolate the view to an active objects and its children.
However removing active object isnt directly related to this patch.